Fluid may be used to manipulate the movement of small particles. One method for controlling the location of a zero-velocity position in flow is described by Bentley and Leal (“A computer-controlled four-roll mill for investigations of particle and drop dynamics in two-dimensional linear shear flows”, J. Fluid Mech., v. 167, pp. 219–240, 1986). The Bentley/Leal device provides four rollers, which rotate at various speeds in specific directions to produce a specific flow type. The device can be used to create a purely extensional flow to manipulate millimeter-size particles; for example, the device may be used to manipulate the behavior of a drop of oil in water, under a force of extensional flow.
The Bentley/Leal device employs a complex computer-controller to keep the center of mass of a particle superposed on the fluid flow stagnation point, while maintaining a specific flow type. To this end, the computer-controller regulates the speed and direction of movement of the four rollers in a tank of fluid.
The Bentley/Leal device has drawbacks and limitations that are not insignificant. For example, the operation of the device depends on a complicated computer-controlled system. Variation in the movement and/or speed of each of the four rollers contributes to the overall behavior of the system. Additionally, the four, relatively large, rollers are moving parts within close proximity of the millimeter-size particle, thereby interfering with, for example, observation of the particle. Further, the rollers in the Bentley/Leal device sit in the same bath of fluid as the sample or particle under observation. With this configuration, the environmental conditions surrounding the sample under investigation (such as the fluid type, ionic strength and/or type, pH, other additives such as specific enzymes, etc.) cannot be altered seamlessly or easily, because the Bentley/Leal device does not provide a means for introducing fluid into the closed bath of fluid.
Another drawback of the Bentley/Leal device is that it employs a relatively deep bath of fluid, resulting in a fluid flow that is non-planar, thereby causing the particle trapped by the flow to drift up and/or down, without leaving the stagnation “point” (or a locus of stagnation points). More particularly, with an optional imaging device located directly above or below the Bentley/Leal four-roll mill device, the trapped particle may drift out of focus, especially during prolonged observation.
Therefore, there exists a need for improved methods and/or systems for confining an object of interest in a region of fluid flow. There is also a need to confine an object in the region for an indefinite length of time and without the aid of an optical trap, a micropipette, or other tethering device. Furthermore, there is a need for systems and/or methods for trapping of an object in bulk solution, sufficiently distant from walls or stationary objects that may interfere with the state or behavior of the object.